Auxin Plant Growth Regulaors in Horticulture

Presentation on Plant Growth Regulator specially Auxin in commercial uses

1. AUXIN & its Commercial uses in Horticultural crops

2. General plant hormones Auxins (cell elongation) Gibberellins (cell elongation + cell division - translated into growth)  Cytokinins (cell division + inhibits senescence)  Abscisic acid (abscission of leaves and fruits + dormancy induction of buds and seeds) Ethylene (promotes senescence, epinasty , and fruit ripening) 

3. Internal and external signals that regulate plant growth are mediated, at least in part, by plant growth-regulating substances, or hormones (from the Greek word hormaein , meaning "to excite"). Plant hormones differ from animal hormones in that:  No evidence that the fundamental actions of plant and animal hormones are the same. Unlike animal hormones, plant hormones are not made in tissues specialized for hormone production.

4. Unlike animal hormones, plant hormones do not have definite target areas. Auxins can stimulate adventitious root development in a cut shoot, or shoot elongation or apical dominance, or differentiation of vascular tissue Substances produced in small quantities by a plant, and then transported elsewhere for use have capacity to stimulate and/or inhibit physiological processes.

5. The Auxin : Cytokinin Ratio Regulates Morphogenesis in Cultured Tissues High auxin:Cytokinin ratios stimulated the formation of roots Low auxin:Cytokinin ratios stimulated the formation of shoots And intermediate levels the tissue grew as an undifferentiated callus ( Skoog and Miller,1965) The effect of auxin:cytokinin ratios on the morphogenesis can also be seen in crown gall tumors by mutation of the T-DNA of the Agrobacterium Ti plasmid

6. Auxin Class Auxin Endogenous Hormone Indoleacetic Acid Growth Regulators IBA, NAA, 2,4-D

7. Auxin -like growth regulators I ndolebutyric acid (IBA) 2, 4 D ichlorphenoxyacetic acid (2,4D) 2, 4, 5 T richlorophenoxyacetic acid (2, 4, 5 T) Picloram

8. Auxin ( indoleacetic acid) C ell elongation and expansion S uppression of lateral bud growth I nitiation of adventitious roots S timulation of abscission (young fruits) or delay of abscission H ormone implicated in tropisms (photo-, gravi -, thigmo -)

9. Plant has apical bud removed so A xillary buds grow

10. Auxins Auxin increases the plasticity of plant cell walls and is involved in stem elongation. Asymmetrical placement of cut tips on coleoptiles resulted in a bending of the coleoptile away from the side onto which the tips were placed (response mimicked the response seen in phototropism). Frits Went (1926) determined auxin enhanced cell elongation.

11. Discovered as substance associated with phototropic response. Occurs in very low concentrations. Isolated from human urine, (40mg 33 gals -1 ) In coleoptiles (1g 20,000 tons -1 ) Differential response depending on dose.

12. Auxin promotes activity of the vascular cambium and vascular tissues . plays key role in fruit development Cell Elongation Acid growth hypothesis auxin works by causing responsive cells to actively transport hydrogen ions from the cytoplasm into the cell wall space

13. Synthetic Auxins W idely used in agriculture and horticulture prevent leaf abscission prevent fruit drop promote flowering and fruiting control weeds Agent Orange - 1:1 ratio of 2,4-D and 2,4,5-T used to defoliate trees in Vietnam War. Dioxin usually contaminates 2,4,5-T, which is linked to miscarriages, birth defects,leukemia , and other types of cancer. 

14. Additional responses to Auxins A bscission - loss of leaves F lower initiation S ex determination Fruit development A pical dominance

15. Commercial uses - A uxins Applied as IBA, NAA, auxin -conjugates or mixes S timulation of adventitious rooting 2,4-D as a herbicide for dicots Sprout prevention in pruned trees F ruit thinning or fruit holding depending on stage of development

16. Auxin is the only plant hormone known to be transported polarly . Polar transport is basipetal (toward the base). Auxin Provides Chemical Signals That Communicate Information Over Long Distances. Apical Dominance- inhibitory influence of the apical bud upon lateral buds.

17. Auxins stimulate cell enlargement Shown when coleoptile (oat seedling stem) segments are floated on IAA Does not work on intact seedlings (they contain enough endogenous IAA) Auxin is required to start cell growth in tissue culture (with cytokinins ) increases cell extensibility A ctivates ATPases that excrete H+ into apoplasm (cell wall) via signal transduction chain Activates expansin activity (loosens X-links) Turgor pressure allows cellulose fibrils to slide past each other and the cell to grow

18. Auxin action over gene activation Auxin causes acid growth in the short term Need to activate transcription of IAA sensitive genes in the long term Fastest response: short auxin upregulated RNAs Slightly later: IAA genes whose transcription is normally prevented by regulatory protein:DNA interactions Auxin may induce ubiquitin binding and proteasome mediated degradation of these proteins All of these genes are thought to induce transcription of genes necessary for growth and development

19. Auxin Plays a Role in the Differentiation and Regeneration of Vascular Tissue

20. Auxin Promotes Fruit Development

21. Auxin Promotes the Formation of Lateral and Adventitious Roots

22. Auxin Prevents Abscission

23. Synthetic Auxins Kill Weeds- Herbicides

24. Some known actions; Establishment of polarity of root-shoot axis during embryogenesis Cell elongation Cell differentiation Apical dominance Lateral root formation and adventitious root formation Fruit formation

25. Evidence for the role of auxin in formation of fruit and structures of similar function Without seed formation, fruits do not develop. Developing seeds are a source of auxin . Auxin replacement restores normal fruit formation and can be used commercially to produce seedless fruits. However, too much auxin can kill the plant and thus synthetic auxins used commercially as herbicides